Pre heat oven to 375 degrees.,______,In separate saucepot, melt butter. Add flour, salt, black pepper and cayenne pepper. Mix Well.,Cook on medium heat one minute while stirring.,Gradually add milk, stirring until well blended.,Add cheese.,Cook until chesse is melted, stirring constantly.,Lay potatoes and onions in 9 x 13 inch baking dish.,Cover with cheese sauce.,Cover with foil.,Bake for 45 - 50 minutes, removing foil after 25-35 minutes.